For the VOIP application, sometimes the ISP will ask you to disable SIP ALG on your modem router. Please refer to the following steps to do the configuration.
Step 1 Refer to the link below to log in to the web management interface.
How to log into the web-based interface of the AC VDSL/ADSL Modem Router
Step 2 Go to Advanced > NAT Forwarding > ALG to turn off SIP ALG.
